The invention relates to the general field of electronic devices provided with antennas, for example devices of the RFID (“radio frequency identification”) type equipped with near-field communication antennas.
Near-field communication antennas of RFID type devices include a plurality of turns formed by conducting paths connected to a microcircuit (currently designated by persons skilled in the art with the expression “RF chip”). RFID devices designed for near-field communication are generally configured for near-field communication in frequency bands comprised between the low frequencies (for example 125 kHz) and high frequencies (for example 30 MHz). RFID devices can be based on the ISO 14 4443 standard or the ISO 15693 standard.
To obtain a desired operation, it is necessary to have good control of the geometric parameters of the antenna so that the associated resonance frequency is well adjusted. The resonance frequency is selected to correspond to that of an external reader. The resonance frequency is calculated taking into account the electrical parameters of the microcircuit of the device.
Moreover, the antenna must be configured to receive enough of the magnetic flux of the magnetic field of the external reader, and the effective surface comprised between the turns of the antenna is adjusted for this purpose. The energy received by means of this magnetic flux is then supplied to the microcircuit, with losses that correspond to the resistance of the coil forming the antenna.
The elaboration of an RFID device designed for near-field communication is therefore performed on the basis of a compromise between the size of the antenna, the resistance losses and the resonance frequency. These parameters are selected to allow the reception of enough energy and the achievement of good performance.
There exists a strong demand for small-sized RFID devices. In numerous application such as for example connected objects, it is particularly critical to reduce the dimensions of the RFID devices, which can appear in the form of a module which can be incorporated in an object. For example, this object can be an enclosure or a bracelet in which the module is directly incorporated.
Also, the module can be arranged between two layers of plastic to form a small-format card which will subsequently be integrated into an object. By small format is meant a format the surface area of which is substantially equivalent to or less than a fourth of the surface area of an ID-1 format (85.60 mm*53.98 mm) according to the ISO7810 standard.
According to the prior art, the adjustment of the electrical parameters of the antenna is accomplished, regardless of the geometric constraints of the antenna, by compensation by connecting a capacitor to this antenna to increase the capacity of the resonant circuit comprising the antenna.
Numerous capacitor integration solutions have been proposed. In particular, it has been proposed to use surface-mounted components or planar capacitors printed directly on the surface of a printed circuit card or on a substrate.
Planar capacitors offer a good level of robustness, their capacitance value is easily calculated, and this value is easily controlled in a production chain. In fact, the capacitance value of a planar capacitor with parallel plates or electrodes is equal to:
                    ϵ        0            ⁢              ϵ        r            ⁢              A        C              d    ,in farads
With ϵ0 the permittivity of vacuum, ϵr the relative permittivity of the material separating the two electrodes of the capacitor, Ac the area of the facing electrodes with respect to one another, and d the distance separating the electrodes.
FIG. 1 is a top view representation of a module 1 according to the prior art, provided with an antenna and a capacitor. Such a module can be integrated into an RFID device. Here the module 1 is formed on a base 2, for example a dielectric substrate. A capacitor electrode 3 and an antenna 4 have been formed on the base 2.
The capacitor electrode 3 is a planar electrode printed on the base 2 in the form of a rectangle disposed substantially at the center of the module 1. The other electrode of this capacitor is disposed on the face of the base opposite to that visible in FIG. 1.
The antenna 4 includes turns printed around the electrode 3. Other turns of the same antenna can be printed on the opposite face.
Moreover, crossing conductive connections 6 are formed through the base, and the antenna 4 and the capacitor 3 are connected to a microcircuit 5.
This arrangement according to the prior art in which the antenna turns are positioned in the periphery, is used to increase the effective surface area of the turns of the antenna.
Generally, the relative permittivity for dielectric materials used in RFID devices is comprised between 3 and 9. The capacitance value of the capacitors therefore depends mainly on the ratio between the area of the electrodes and the distance that separates them.
Due to the size limitations imposed on these devices, it is difficult to increase the area of the electrodes.
This problem can be resolved by selecting a suitable dielectric material and a thin substrate (the electrodes of the capacitors are disposed on the opposite faces).
This being the case, the thinnest substrates, having for example a thickness smaller than 75 micrometers, are complicated to handle. This is the case of substrates made of polyethylene terephthalate (PET) or of polyimide (Kapton) which can be on the order of 40 micrometers. The steps of cutting or of placement are particularly difficult to implement on such substrates.
As can be imagined, placement errors can lead to the cutting of an antenna during a subsequent cutting step, for example for an antenna such as that illustrated in FIG. 1.
In FIG. 1, the placement error for a cutting step is limited to a value δ0 which is the smallest distance between the largest turn (or exterior turn) and an edge of the module. This value is generally too small and unsatisfactory. This value δ0 corresponds to the allowable margin of error concerning the placement of the module during a cutting step, and therefore to a maximum cutting tolerance value. Currently, this value is less than 500 micrometers for thin, soft substrates made of polyimide.
The invention aims in particular to mitigate these disadvantages, and in particular to reduce losses during manufacture of devices comprising an antenna and a capacitor.